/Engine/Plugins/Interchange/Runtime/Source/Parsers/GLTFCore/Public/GLTFMaterial.h
"pbrSpecularGlossiness" extension Implements a specular glossiness shading model and is mutually exclusive with "specular" extension, which uses metallic roughness model
struct FSpecularGlossiness
float
GlossinessFactor
FTextureMap
Map
FVector
SpecularFactor